Metalorganic chemical vapor deposition (MOCVD) of Al2O3 was studied by using trimethylaluminum and O-2 precursors over a variety of temperature, flow, and pressure conditions. Growth within the low temperature region from 600 to 800 degrees C was found to be limited by a surface kinetic process that follows the Langmuir-Hinshelwoocl mechanism. In the high temperature region from 900 to 1050 degrees C, the growth process was influenced by prereactions in the gas phase. The crystallinity of MOCVD Al2O3 films grown in situ on GaN substrates was characterized by grazing incidence x-ray diffraction (GIXRD) and high-resolution transmission electron microscopy (HRTEM). GlXRD suggested that the Al2O3 films grown at 600 and 700 degrees C were amorphous, while those grown at 800, 900, 1000, and 1050 degrees C were crystalline with primarily gamma-phase. HRTEM further showed that the gamma-Al2O3 films grown at 900 and 1050 degrees C were poly and single crystalline, respectively. The crystal orientation relationships during the epitaxial growth of 7-Al2O3 on GaN at 1050 degrees C were also determined. (C) 2014 Elsevier B.V. All rights reserved